Hi, Ciarrai, good question. My better works seem to happen in short spurts of a couple of hours rspaced over two or three weeks. I need lots of time away from the picture before I can spot what I want to change.
Right now I have a job that leaves me three days off each week. Of course not all of that time can go on painting as I occasionally have to kick the dirt in the house back a couple of feet, etc., so short work sessions have become my habit. It will take about three of those "weekends" to get something more or less completed. I usually have only one main picture going but lots of little sketches and experiments which mostly end up in the garbage.
I suppose it only adds up to one or two 8-hr days for a pastel painting, much the same for oil. I do fast watercolours but they're all terrible. I work in layers, first blocking things in very tentatively, then making it more defined, puzzling over the composition, etc. Often, after I've put it away for a few weeks, supposedly finished, I see what it really needs and put in a few final strokes. As a consequence, my pictures are only declared "done" when I have to take them to the framer's, and I often forget to photograph them or even sign them before the glass goes on.
I know lots of artists have a clear idea from the beginning and work very methodically, sometimes getting quick and impressive results, other times working hours each day on a detailed work with infinite patience. This isn't me. I don't take a long time because I'm meticulous (I have poor vision, no eye for detail, and no patience) but because I'm indecisive and a bit blind to my own work.
I don't think the amount of time you spend is in itself important, the questions is how does your approach influence the work? It's a really personal decision. Some types of painting do benefit from being done quickly with passion, while others need careful crafting. A teacher of mine used to tell the class that when the general public asks how much time we spend on a painting we should always exaggerate it, because some people just can't understand that a good painting can be done quickly by a skilled artist on a good day, while months of work can be wasted on a non-flyer. (I've never followed his advice, but I feel discomfort when people - I don't mean artists - ask how much time it took to do a particular work.)
If you don't like what you're producing, it's possible you should stop at an earlier stage of the picture to evaluate. But if you do like your pictures, don't think you need to spend some preconceived amount of time on them. Being able to turn out quality work quickly is a huge advantage to an artist.